In single phase AC power supplies, the output power is modulated at twice the output frequency. For the typical case of a sinewave, the output power varies from zero to twice the average power:P(t)=PAVG+PAVG·sin(2πft)  (1)
From this equation, a sinusoidal voltage/current output causes the output power to swing from 0 to twice the average output power with a frequency of twice the voltage waveform.
Because of the energy conservation principle, the input power must be equal to the output power plus any energy accumulated in the converter. Modern converters typically use capacitors to temporally store energy. Because of size and cost constrains this energy storage is limited, making it more effective at high frequency than low frequency (longer cycles).
Due to the required energy storage, the generation of low frequency power signals is one of the main factors that determines required internal capacitance. Another key factor is the bus voltage regulation, because it affects how well the available capacitance is used.
When the DC bus capacitance is not big enough or the control loop does not provide proper regulation, the bus voltage can deviate from its ideal DC set-point. Allowing the DC bus in the capacitor bank to swing over a wide band has two significant disadvantages:
1) The difference between the maximum and minimum DC bus voltage represents available energy storage that cannot be really accounted for. This means that the utilization of the capacitors is reduced, thus increasing cost and volume.
2) The power stages that are fed from the DC bus (typically DC to DC converters or inverters) are designed to operate at both the maximum DC bus and the minimum. If this range is too high, it can significantly affect cost, size and energy efficiency. For example, higher voltage FETs have higher conduction losses than lower voltage ones for a given die size, and also a higher DC bus increases switching losses.
Large fluctuations in the output power, such as the ones caused by low frequency AC, cannot be totally filtered by a reasonable amount of capacitance. The only way to avoid large variations in the DC bus is to allow the input power to have similar fluctuations. If the input power is pulsed, or modulated in amplitude, the input power factor is severely affected thus increasing power losses in the input circuits and AC electrical installation. For the extreme case of full modulation of the input power (i.e. no internal energy storage) due to a low frequency sinusoidal output, the theoretical power factor is the following:
                    PF        =                                            P              AVG                                                                        P                  AVG                  2                                +                                  P                                      A                    ⁢                                                                                  ⁢                    C                                    2                                                              =                                                    P                AVG                                                              P                  AVG                                ⁢                                                      1                    +                    0.5                                                                        =            0.8165                                              (        2        )            
Where PAC was assumed to be PAVG/√2 because it was a fully pulsed power signal as in equation (1). This ideal case would be possible with a front end controller that is able to generate the exactly required input power/current based on the output power. This reduction in power factor is too big for practical applications, which typically require power factors higher than 0.9.